Power steering apparatus with a flow control unit

Abstract

A power steering apparatus of the present invention which includes a pump, a power cylinder, a control valve, an orifice arranged in a supply passage leading an operating fluid discharged from the pump to the control valve, a flow control valve, and a switching valve. A spring chamber of the flow control valve is connected to the supply passage downstream of the orifice, and is responsive to the pressure difference across the orifice for regulating the flow rate of an operating fluid to the control valve to an almost constant flow rate. The switching valve is actuated in response to the differential pressure across the orifice for switching the communication of the spring chamber between the downstream of the orifice and the reservoir. When the differential pressure across the orifice is relatively low, the switching valve brings the spring chamber into communication with the reservoir. With this configuration, an excessively high pressure is not maintained in the pump, and an increase in the temperature of the operating fluid which brings about seizure of the pump can be prevented.

What is claimed is: 
     
       1. A power steering apparatus comprising: a pump for supplying operating fluid;   a power cylinder having a pair of fluid chambers and being operable to generate an assist force;   a control valve having variable throttles which are arranged on paths connected to said pump, both fluid chambers of said power cylinder, and a reservoir;   an orifice arranged on a supply passage leading the operating fluid discharged from said pump to said control valve;   a flow control valve which has a spring chamber connected to said supply passage downstream of said orifice and responsive to the pressure difference across said orifice for regulating the flow rate of the operating fluid to said control valve to an almost constant flow rate; and   switching valve responsive to the differential pressure across said orifice for switching the communication of said spring chamber of said flow control valve between the downstream of said orifice and said reservoir.   
     
     
       2. A power steering apparatus according to claim 1, wherein said switching valve brings said spring chamber of said flow control valve into communication with said reservoir when the differential pressure across said orifice is relatively low. 
     
     
       3. A power steering apparatus according to claim 1, wherein said switching valve comprises a first annular groove to which the operating fluid passing through said orifice is led and a second annular groove connected to said reservoir, and wherein said switching valve is switched in response to the differential pressure across said orifice to the position that either said first or second annular grooves is communicated with said spring chamber of said flow control valve. 
     
     
       4. A power steering apparatus according to claim 3, wherein said switching valve is switched to the position that said second annular groove is communicated with said spring chamber of said flow control valve when the differential pressure across said orifice is relatively low.